Counterbalancing sickle drives are known in the art. See for example U.S. Pat. No. 3,941,003 owned by the assignee of the present invention. In that patent, a pair of equal counterbalance weights rotate in opposite directions about the axis of the main shaft of a gearbox that supplies driving power to the sickle. Each time the sickle approaches one end of its path of travel, the counter-rotating weights come into superimposed relationship with one another at the opposite end of the path of travel so as to counteract the inertia of the sickle. While this arrangement does a good job of counterbalancing the reciprocating sickle, it fails to compensate for the centrifugal force generated by an oscillating sway bar used in the translation of rotary motion from the gearbox to reciprocal motion of the sickle.
It is also known in the art to use an epicyclic drive for reciprocating a sickle. This entirely eliminates the need for an oscillating sway bar. However, until the present invention, no epicyclic sickle drive having the ability to counterbalance the reciprocating sickle has been available.
Accordingly, one important object of the present invention is to provide an epicyclic sickle drive that is capable of counterbalancing the sickle it reciprocates. In one preferred embodiment, the drive includes a crankshaft that orbits about the axis of a main shaft as the main shaft rotates, while simultaneously rotating oppositely from the main shaft. A drive stud on the crankshaft is offset from the axis of the crankshaft to such an extent that the stud simply moves back and forth in a straight line path of travel as the crankshaft orbits about the main shaft, thereby reciprocating the sickle that is bearing-coupled to the stud. The main shaft has a first counterbalance weight secured thereto that rotates with the main shaft during its operation. A second counterbalance weight is fixed to the crankshaft so that the crankshaft weight rotates oppositely from the main shaft weight. While the center of gravity of the main shaft weight moves at a constant velocity in a circular path of travel about the axis of the main shaft, the center of gravity of the crankshaft weight moves in an elliptical path of travel about that axis. Thus, the center of gravity of the crankshaft weight decelerates and accelerates adjacent opposite ends of its elliptical path of travel, creating a condition that must be compensated for by the main shaft weight. Accordingly, the main shaft weight is heavier than the crankshaft weight by an amount that produces the desired compensation.